Organ circuit



J. R. BRAND 3,415,941

one/m cmcun Filed lay 24, 1965 M MODULATOR m) 11 I 76 3 5+ I 23 Z JAll/(167143141 5% M Y REPETITION J RATE 15+ I9 27 22 $9 27 N 28 M jINVENTOR. 3 BYJQHN R. BRAND ATTORNEY United States Patent 3,415,941ORGAN CIRCUIT John R. Brand, Northrirlge, Calif., assignor to WarwickElectronics Inc., Chicago, Ill., a corporation of Delaware Filed May 24,1965, Ser. No. 458,198 12 Claims. (Cl. 84-1.26)

ABSTRACT OF THE DISCLOSURE The tone envelope of a musical instrument iscontrolled by a storage capacitor. The attack of the envelope isdetermined by the charging resistance of the capacitor circuit. Thedecay or sustain of the envelope is determined by the dischargeresistance. Either or both resistances are in eifect adjusted by thepresent invention by rapidly and alternately blocking and passingcurrent through the resistance at a controlled and adjustable dutycycle. When the duty cycle is high, so that the circuit is closed duringa high percentage of the time, the current flow is high and theresistance appears low. When the duty cycle is adjusted to be low, thecurrent is correspondingly low and the apparent resistance is high.Thus, the apparent value of the resistance in the RC circuit is readilyand adjustably controlled. The invention has the unique ability ofrequiring only one duty cycle control, preferably in the form of anadjustable multivibrator, for a large number of discharge (or charge)resistors.

This invention relates to a modulation circuit for an electronic musicalinstrument such as an electric organ, and has particular reference to acircuit for adjustably controlling the attack and/or decay of apercussion tone of the organ.

It is an object of this invention to provide an adjustable decay (orattack) for an organ tone in which the nature of the decay (or attack)is substantially the same irrespective of how many notes might besimultaneously played on the organ.

It is another object of this invention to provide means for adjustingthe charge or discharge characteristic of the control capacitor of apercussion circuit without changing the potential to which the capacitorcharges or discharges as the case may be.

It is a further object of this invention to provide, in a percussion orsustain circuit for an electric organ, a simple means for achieving theeffect of adjustable decay (or attack) without actually changing anyresistance or impedance value in the circuit.

It is a further object to provide simplified means for controlling thedecay (or attack) characteristics of a plurality of percussion orsustain circuits as employed in a polyphonic instrument.

In accordance with these and other objects which will become apparenthereinafter, preferred forms of the present invention will now bedescribed with reference to the accompanying drawing wherein:

FIG. 1 is a circuit diagram of the present invention applied to thesustain or decay of an organ percussion tone.

FIG. 2 illustrates the application of the present invention to theattack of an organ tone.

FIG. 3 is a modification of the circuit of FIG. 2 to overcome certainpossible shortcomings thereof.

Referring to FIG. 1 there is represented at 11 a tone signal source ofgiven pitch. The organ comprises a plurality of such sources ofdifferent pitch, usually corresponding to the chromatic musical scale.Tone signal output from the source 11 is fed to a modulator or gate 12where the shape of the tone envelope is determined by 3,415,941 PatentedDec. 10, 196E the potential on a control treminal 13. Modulated tone ithen fed to an output system 14, usually common to a] of the tonesources, and which usually includes soml form of electro-acoustictransducer symbolized by thl loudspeaker 16.

Normally, the modulator 12 is cut off so that tone signal does not passto the output system 14. Applicatior of control potential to theterminal 13 in effect unblock: or opens the modulator 12 and allows thetone to be sounded. The configuration of the tone envelope is determinedby the wave shape of the control potential. Potential on the terminal 13is derived from charge on a capacitor 17 connected to the terminal 13.Capacitor 17 is charged from a source of positive potential 19 through anormally open key switch 21 and charging resistor 22. The key switch 21is linked to a keyboard key 23, which may be manual or pedal.

As long as the key switch 21 is held closed, current flows through theresistor 22, resistor 26, and diode 27 to a common bus 28. Current mayor may not in fact flow through 26, depending upon the potential of thebus 28, as will be discussed hereinafter. To the extent that currentdoes flow through the resistor 26, the final potential on capacitor -17is determined by the voltage division between resistors 22 and 26. Ifthere is no current flowing to the bus 28, the full potential from theterminal 19 appears on the capacitor 17 through the RC charging circuitincluding the resistor 22. This potential opens the modulator 12 andallows tone signal from the source 11 to pass to the output system 14.

In some cases it may be more desirable to avoid the voltage division.This may be done by connecting the left hand side of resistor 26 to theleft hand side of resistor 22, instead of the right hand side.

When the key 23 is released, opening the switch 21, modulator 12 doesnot instantly cut off, because of the residual charge on capacitor 17.The decay of tone through the modulator 12 is determined by thedischarge characteristic of the capacitor 17.

The discharge circuit for the capacitor 17 consists of the resistor 26,diode 27, common discharge bus 28, and thence to a circuit representedby the block 31, which alternately and repetitively or cyclically placesthe bus 28 at the B plus potential of the terminal 19, or grounds thebus 28. This function of the circuit 31 is represented symbolically by adouble-throw switch 32, one contact of which is connected to a B plussource 33, and the other which is grounded at 34. Since the capacitor 17is also grounded at 36, the switch 32, when in its lower position,serves to complete a discharge circuit for the capacitor 17.

The discharge circuit for the capacitor 17, consisting of the resistor26, diode 27, bus 28 and switch 32, contsitutes in effect a gate meanswhich is repetitively and alternately blocked and unblocked. Switch 32,when connected to the terminal 33, represents the blocked condition,because the diodes 27 are reverse biased and therefore current cannotflow. Switch 32 in the lower most position represents the unblockedcondition, for then there is a discharge path through the resistor 26,diode 27, bus 28, switch 32, to ground 34. The RC dischargecharacteristic of the capacitor 17 is, under those conditions,determined primarily by the capacitance of 17 and the resistance of 26(assuming negligib e forward resistance in 27). During those portions ofthe cycle of switch 32 when current is flowing, the capacitor 17 isalways discharging to the same potential, namely the ground potential 34placed on the bus 28. By varying the duty cycle of the switch 32, so asto control the time ratio between the grounded condition and the B pluscondition of the bus 28, the average discharge current can becontrolled, with an effect equivalent to that of controlling theresistance of the discharge path. The

range of this control varies from an infinite resistance, (were theswitch 32 to be permanently fixed to the terminal 33), down to the valueof the resistance 26, (were the switch 32 to be permanently fixed to theground terminal 34).

The variable duty cycle is effected by employing a multivibrator havingan adjustable or controllable repetition rate. This is representedschematically by the block 36, the control of the repetition rate beingillustrated schematically by the adjustable resistor 37. It is to beunderstood that the blocks 31 and 32 are, in actual circuitry, bothcombined into a single circuit denominated here as multi-vibrator means.The net effect of the multi-vibrator, however achieved, is torepetitively or cyclically apply B+ and ground to the common bus 28.This is represented by the wave form shown at 38. The .adjustability isachieved by any means which controls the ratio between the B plus timeand the ground time of the bus 28. This may be done by adjustablycontrolling the width of the pulses 41, or may be done, and preferablyis done, by a multi-vibrator which delivers constant-width pulses 41 ata variable repetition rate.

The diode 27 forms a portion of the gate circuit and is valuable in thecircuit to isolate one note from the other. The term diode as usedherein refers to any current-conducting element having a curve relationbetween voltage and current that possesses two markedly differentimpedances. In the example shown, assume that the voltage on theleft-hand side of the diode 27 is negative with respect to the righthand side or terminal of the diode. Under these circumstances the diode27 exhibits high impedance. As the voltage on the left hand terminal ofthe diode 27 goes more positive relative to that on the right handterminal, a point or region is reached where the impedance abruptlydrops. In a typical diode, this region is not a point but represents arounding or knee of the curve, although ideally it would occur abruptly.Also, in an ideal diode this point would occur exactly where the voltageon the left hand terminal becomes positive relative to that on the righthand terminal. As a practical matter, the voltage on the left handterminal must, in fact, become somewhat more positive than that on theright hand terminal before the knee of the curve is encountered.

With this understanding of the term diode as employed herein, it will bereadily seen that when the discharge bus 28 is at B plus potential, thediode 27 is reverse biased, presenting a very high impedance-virtuallyan open circuit-to the discharge of the capacitor 17. When the bus 28 isgrounded, the diode 27 is forward biased by virtue of the differentialin potential between the voltage on the capacitor 17 and the groundconnection made to the bus 28. This presents a very low impedance tocurrent flow.

The circuit described above is repeated for each of the various notes,except, however, that the bus 28 and the associated circuits 31 and 36,and the output circuit 14 are common to all (or a large group) of thenotes.

A striking advantage of the present invention is that the decay orsustain characteristic of the tone is independent of the number of keyswitches 21 closed at any one time. If the effect were to be achieved byactually varying a common resistance in the RC discharge circuits, thedecay characteristic would shift depending on the number of notes beingplayed at any one time, because the discharge current would changethrough the common impedance or resistance, and thus affect the decaycharacteristic of the tone. For this reason, it is important that theresistance between the common bus 28 and the lower or ground side 36 ofthe capacitor 17 be as low as feasible during those portions of thecycle when the multi-vibrator represented by the switch 32 is in itslower position. On the other hand, the impedance or resistance betweenthe common bus 28 and the source 33 of B plus voltage is of littlemoment, because no current is flowing during that condition.

The multi-vibrator represented by the blocks 31 and 36 may assume any ofa number of well-known forms to achieve the function describedhereinbefore. It may, in fact, be a physical switch shown at 32, auni-junction multi-vibrator, or any other suitable oscillating circuitryto produce the prescribed conditions. It is, of course, necessary thatthe pulse or repetition rate be high relative to the time of themodulation envelope effected in the modulator 12. That is to say, therate should be sufiiciently high so that the capacitor 17 appearsvirtually as a short circuit at those frequencies.

The principles of the present invention may also be employed toadjustably control the attack of a tone passing through the modulator12. This is illustrated in FIG. 2, wherein like numerals have beenemployed to represent like elements. In FIG. 2 there is a commoncharging bus 51 to which is selectively and alternatively applied, byswitch 56, B plus from the terminal 19 or ground from the groundconnection 52. This intermittently applied B plus is transmitted to thecapacitor 17 through a diode 53, the key switch 21, and the chargingresistor 22.

The operation of circuit of FIG. 2 is substantially the same as thatdescribed in connection with FIG. 1. The higher the pulse repetitionrate, illustrated by the pulse stream 54, the lower will be the apparentor effective charging resistance. In the extreme, if the switch 56 isfixed to the B plus terminal 19, the charging resistance is simply theresistance 22. If the switch 56 were fixed to ground 52, the chargingresistance would, in effect, he infinite, because there would be nopotential tending to charge the capacitor 17 In between these twovariables, as determined by the pulse repetition frequency, the chargingresistance can be made any apparent value desired. This, of course, willcontrol the shape and nature of the attack of the envelope of the tonepassing through the modulator 12.

The circuit of 'FIG. 2 is subject to a possible weakness in that thedischarge resistor 26 is permanently connected to the capacitor 17.Thus, if the sustain or decay timing is set very short, it is possiblethat the capacitor 17 might never attain its full designated charge,because the moment a charge appears on the capacitor 29 it starts toleak off or disappear through the discharge circuit, including theresistor 26. Thus, the circuit of FIG. 2 would be unsatisfactory foroperation with a slow or long attack and a fast or short sustain ordecay.

This weakness may be remedied by the circuit shown in FIG. 3, whereinthere is employed for .the key switch, instead of a single-throw switch21, a double-throw switch 21a. In the off position, i.e. when the note23 is not played, the sustain capacitor 17 is connected to the dischargeresistor 26. When the note 23 is played, the switch 21a is connected tothe charge circuit, or charging bus 51. Thus, by use of a double-throwswitch, the charge and discharge circuits are mutually isolated, so thatthe attack and decay adjustment are completely independent of eachother.

It will be obvious that the present invention can also be applied to aninstrument in which the playing of a key institutes discharging, ratherthan charging, of a capacitor, to cause tone to sound.

Furthermore, it will be recognized that this invention can be applied toa tone generator having a frequencystable output whose amplitude isproportional to a control voltage, in this case, the voltage appearingat 13. In such case the tone generator would replace the tone source 11and modulator 13 shown in FIG. 1.

While the instant invention has been shown and described herein in whatis conceived .to be the most practical and preferred embodiments, it isrecognized that departures may be made therefrom within the scope of theinvention which is therefore not to be limited to the details disclosedherein, but is to be afforded the full scope of the invention ashereinafter claimed.

What is claimed is: 1. Circuit for controlling the shape of the toneenvelope of an electronic musical instrument, comprising:

capacitive means and resistive means connected together,

means for causing a current to flow through said capacitive andresistive means,

circuit means for modulating a tone signal in accordance with thevoltage on said capacitive means,

gate means forming at least a portion of said resistive means,

free-running means for repetitively blocking and unblocking said gatemeans at a frequency which is high relative to the time of modulation ofthe tone signal, and

means for adjusting the time ratio between blocked and unblockedcondition of said gate means,

thereby to control the average value of said current, whereby theapparent resistance of said resistive means, to direct current, iscontrolled.

2. Circuit for controlling the sustain time of a note in an electronicmusical instrument comprising:

a capacitor,

means for charging said capacitor in response to playing of a note,

means responsive to charge on said capacitor for enabling a tone .to besounded by the instrument,

a discharge circuit for said capacitor,

release of a note serving to institute discharge of said capacitorthrough said discharge circuit,

gate means for forming at least a portion of said discharge circuit,

means for repetitively blocking and unblocking said gate means, and

means for selectively controlling the time ratio between blocked andunblocked condition of said gate means,

thereby to control the apparent DC resistance of said discharge circuit.

3. Sustain control circuit for an electric organ having a plurality oftone sources, a corresponding plurality of playing keys effective tocause tones from said tone sources to be sounded, and a correspondingplurality of capacitors chargeable by playing of said keys to maintainthe sound of the tones in response to the charge thereon, characterizedby:

a discharge bus common to said capacitors for transmitted dischargecurrent from said capacitors,

a plurality of discharge circuits corresponding to said capacitors fortransmitting discharge current to said discharge bus,

gate means connected to said bus for completing a discharge path forsaid capacitors,

free running means for intermittently blocking and unblocking said gatemeans, and

means for controlling the time ratio between blocked and unblockedcondition of said gate, thereby to control the average currenttherethrough and thus control the discharge rate of the capacitors.

4. Circuit for controlling the shape of the tone envelope of anelectronic musical instrument having:

capacitive means and resistive means connected together,

means for causing a current to flow through said capacitive andresistive means, and circuit means for modulating the tone signal inaccordance with the voltage on said capacitive means, characterized by:

diode means forming at least a portion of said resistive means, saiddiode means, when reverse biased serving to substantially block passageof current therethrough, and when forward biased serving to pass currenttherethrough,

free-running means for repetitively reverse biasing and forward biasingsaid diode means at a frequency which is high relative to the time ofmodulation of the tone signal, and

means for adjusting the time ratio between rever: bias and forward biascondition of said dioc means,

thereby to control the average value of said cu1 rent, whereby theapparent resistance of sai resistive means to direct current iscontroller 5. A sustain control circuit for an electric organ havin aplurality of tone sources, a corresponding plurality 0 playing keyseffective to cause tones from said ton sources to be sounded, and acorresponding plurality o capacitors chargeable by playing of said keysto maintaii the sound of the tones in response to the charge thereoncharacterized by:

a discharge bus common to said capacitors for trans mitting dischargecurrent from said capacitors,

a plurality of diodes corresponding and connected respectively to saidcapacitors for transmitting dis charge current from the capacitors tosaid discharge bus,

said diodes, when reversed biased by application of reverse biaspotential to said discharge bus, serving to substantially block passageof discharge current through said diodes, and when forward biased, byapplication of forward bias potential to said bus, serving to permitpassage of discharge current to said discharge bus,

bias means for intermittently and cyclically applying a reverse biaspotential and a forward bias potential to said discharge bus, thereby tointermittently block and enable passage of discharge current throughsaid diodes, and

means for controlling the time ratio between reverse bias and forwardbias on said discharge bus, thereby to control the average dischargecurrent through said diodes and thus control the discharge rate of saidcapacitors.

6. Control circuit in accordance with claim 5 wherein:

said bias means includes multi-vibrator means for developing forwardbias pulses of substantially constant pulse width, and

said controlling means includes means for controlling the pulserepetition rate of said multi-vibrator means.

7. Control circuit in accordance with claim 5 including:

a plurality of charging circuits, one for each said capacitor, eachcharging circuit having therein a key switch operable by a respectivesaid playing key,

a source of charging voltage connected and common to each of saidcharging circuits for supplying charging potential for said capacitors,and wherein:

said bias means cyclically applies to said discharge bus, a potentialsubstantially at least as large as said charging potential, and apotential substantially less than said charging potential,

whereby discharge current is substantially blocked because of absence ofeffective potential difierence when said discharge bus is at said firstmentioned bias potential, and flows to said substantially less potentialduring those portions of the bias cycle when the bias is at saidsubstantially less potential.

8. Circuit in accordance with claim 7 wherein the return impedancebetween said common bus and said capacitors is very low during thoseportions of the bias cycle when the diodes are forward biased.

9. Circuit for controlling the shape of the tone envelope of anelectronic musical instrument, comprising:

storage circuit means,

means for modulating a tone signal in accordance with the energy storedin said storage circuit means,

gate means forming at least a portion of said storage circuit means,

free running means for repetitively blocking and unblocking said gatemeans, and

means for selectively controlling the time ratio between blocked andunblocked condition of said gate means,

thereby to control the average rate of change of energy stored in saidenergy storage circuit means.

10. Circuit for controlling the shape of the tone envelope of anelectronic musical instrument having a plurality of tone sources, aplurality of playing keys, a plurality of storage circuit means, meansfor storing energy in said storage circuit means, and means formodulating tone signals from said tone sources in accordance with theenergy stored in said storage circuit means, characterized by:

a bus common to a plurality of said storage circuit means,

gate means connected to said bus for changing the energy stored in saidstorage circuit means,

free running means for repetitively blocking and unblocking said gatemeans, and

means for selectively controlling the time ratio between blocked andunblocked conditions of said gate means, thereby to control the rate ofchange of energy in said storage means.

11. An adjustable circuit comprising:

impedance means,

voltage means for passing a current through said impedance means,

circuit-breaking means for selectively passing and 5 blocking saidcurrent through said impedance means, free-running means for actuatingsaid circuit-breaking means to alternately pass and block said currentat a repetition rate which is very high with respect to the frequency ofany variation in the voltage of said voltage means, and

means for selectively adjusting said free-running means, thereby toadjust the ratio between passing and blocking time of saidcircuit-breaking means,

whereby theapparent impedance of said impedance means is made to varyinversely with the duty cycle of current passing condition, imposed bysaid freerunning means on said circuit-breaking means.

12. Circuit in accordance with claim 11, wherein:

said impedance means comprises a resistor, and including additionally acapacitor connected to said resistor to form therewith an RC circuit,

whereby adjustment of said free-running means controls the time constantof said RC circuit.

References Cited UNITED STATES PATENTS Re. 25,515 1/1964 Peterson 84-1262,486,208 10/1949 Reinstra 84l.26 2,783,672 3/1957 Hanert 84-1.262,828,659 4/ 1958 Hanert -1 84-126 ARTHUR GAUSS, Primary Examiner. H. A.DIXON, Assistant Examiner.

